The investigation found that quotations given by four of the main Northern Irish banks for a five-year, £5,000 loan gave repayment figures that automatically included Payment Protection Insurance (PPI), rather than offering it to customers as an optional extra.

The Financial Services Authority's (FSA) rules state that banks must offer quotes for loans with and without PPI, and clarify whether or not it is a mandatory stipulation in taking out the loan.

The TSS ascertained that premiums inclusive of PPI could potentially add as much as £1,200, or 16 per cent, to the total repayment cost.

Speaking on behalf of TSS, Jimmy Hughes said: "The investigation gives cause for concern. When the insurance premiums are included in the initial quotation, consumers may not be aware that they are incurring optional and unnecessary costs.

"Although this type of insurance can be useful, it does add substantially to the total repayment figure. Consumers need to be aware of what they are purchasing [and] that way…can make an informed choice based on their individual circumstances."